1. Field of the Invention
The invention relates generally to isolation assemblies used in fracturing/gravel packing, or “frac pack,” systems.
2. Description of the Related Art
Because hydrocarbon production wells are often drilled into unconsolidated formations, sand and fines from those formations will tend to enter the production tubing along with the produced fluids. To prevent this, it has become relatively standard practice to run a fracturing and gravel packing treatment, commonly referred to as a “frac pack,” within the wellbore prior to production. During the fracturing treatment, the production zone is stimulated by creating fractures in the formation rock and flowing proppant material into the fractures to keep the fractures from closing. During the gravel packing operation the annulus surrounding the screen assembly is filled with gravel or another granulated material. This material forms a barrier around the screen and provides a filter to help prevent sand and fines from the formation from entering the production tubing string.
A conventional frac pack system includes a screen assembly that is placed in the wellbore near the unconsolidated formation. The screen assembly radially surrounds a wash pipe, and both the screen assembly and wash pipe are connected, at their upper ends, to a service tool. The usual service tool includes a production packer and a cross-over tool, which are connected to a work string that extends downwardly from the surface. The work string is used to position the screen assembly in the wellbore. Packers provide fluid sealing. The frac pack system can be placed into a “squeeze” configuration, wherein no fluids return to the surface. In this configuration, fracturing fluid is passed through the cross-over tool, into the annulus and then into the formation. Alternately, the frac pack system can be placed into a “circulation” position to allow flow through the wash pipe back to the surface. Gravel packing slurry is then flowed in through the cross-over tool to gravel pack the annulus around the screen assembly. When gravel packing is completed, the service tool is detached from the screen assembly and withdrawn from the wellbore, leaving the gravel packed screen assembly and packer in place. Further details concerning the construction and operation of frac pack systems in general are provided in U.S. Pat. No. 6,789,623 issued to Hill, Jr. et al. This patent is owned by the assignee of the present invention and is incorporated herein by reference.
Traditional frac pack systems have utilized an isolation string that is installed inside the production screen at the surface and is controlled in the wellbore by an inner service string. Typically, the isolation string has two or more sliding sleeve valves that are shifted between open and closed positions mechanically by a shifting tool carried on the wash pipe. One problem with the use of wash pipe-based mechanical shifters is that the wash pipe is relatively weak and provides a point of potential failure where it passes through the isolation string. Additionally, it is time consuming, and thus costly, to have to make up a string of wash pipe to operate the sleeve valves.
One alternative to the use of wash pipe to operate the sleeve valves in the isolation string is described in U.S. Pat. No. 6,397,949 issued to Walker et al. In Walker's system, the isolation string uses one or more pressure activated control valves that are moveable between three functional positions: closed-locked, closed-unlocked, and open-unlocked. It is an intended feature of Walker's system to ensure simultaneous opening of all of the valves within the isolation string. Walker contends that, if all the valves did not open at once, a single open valve would eliminate the pressure differential needed to open all of the other sleeves. Thus, Walker's system does not appear to permit conditions of the gravel packing operation to be monitored through the flowbore during the gravel packing operation when all the valves are closed.
Another wash pipe-less system is described in U.S. Patent Publication No. US 2003/0178198 A1 (Turner et al.). In the described system, the isolation string includes a pressure activated control valve and an object activated control valve. These control valves are each operated in a different manner. The object activated control valve is operated using a ball or other object that is dropped into the flowbore. The pressure activated control valve (PACV) is initially run into the wellbore in a closed-locked configuration. When access to a nearby production zone is desired, a predetermined pressure differential is applied between the casing annulus and the internal annulus to shift an inner sleeve in the PACV to a closed-unlocked configuration. Subsequently, the PACV is moved to an open-unlocked configuration by a reduction in fluid pressure.
The present invention addresses the problems of the prior art.